This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In a TDD system, downlink (DL) and uplink (UL) transmission take place in different, non-overlapping time slots. Typically, a transmitted signal in a radio communication system is organized in some form of frame structure, or frame configuration. For example, LTE generally uses ten equally sized subframes 0-9 of length 1 ms per radio frame. In case of TDD, there is usually a single carrier frequency, and UL and DL transmission are separated in time. Because the same carrier frequency is used for UL and DL transmission, both the BS and the UEs need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither DL nor UL transmission occur in order to avoid interference between UL and DL transmission. For LTE, special subframes (e.g., subframe #1 and, in some cases, subframe #6) provide this guard time. A TDD special subframe is generally split into three parts: a DL part (Downlink Pilot Time Slot, DwPTS), a guard period (GP), and a UL part (Downlink Pilot Time Slot, UpPTS). The remaining subframes are either allocated to UL or DL transmission.
In the current networks, UL/DL configuration is semi-statically configured, thus it may not match the instantaneous traffic situation. This will result in inefficient resource utilization in both UL and DL transmission, especially in cells with a small number of users. In order to provide a more flexible TDD configuration, so-called Dynamic TDD (also sometimes referred to as Flexible TDD) has therefore been introduced. Dynamic TDD configures the TDD UL/DL asymmetry to current traffic situation in order to optimize user experience. For a better understanding of the dynamic TDD subframe configurations, FIG. 1 illustrates an example dynamic TDD subframe configuration.
In the illustrated configuration, dynamic TDD provides an ability of configuring some subframes to be “flexible” subframes, for example, subframes 3, 4, 8, and 9. These flexible subframes can be configured dynamically and flexibly as either for UL transmission or for DL transmission. The subframes being configured as either for UL transmission or DL transmission rely on e.g. the radio traffic situation in a cell. Accordingly, it is expected that dynamic TDD can achieve promising performance improvements in TDD systems when there is a potential load imbalance between UL and DL. Besides, dynamic TDD approach can also be utilized to reduce network energy consumption. It is expected that dynamic UL/DL allocation (hence referred in this section to “dynamic TDD”) should provide a good match of allocated resources to instantaneous traffic.
Further, in Layer one (L1) controlled dynamic TDD, whether the flexible subframe is a DL or a UL subframe is decided by the BS or eNB and the UE will follow the UL and DL grant or some indicators from the eNB to judge the subframe is a DL or a UL subframe. If the eNB schedules the UE in the flexible subframe as UL, then the UE will transmit on the subframe as UL. Similarly, if the eNB schedules the UE in the flexible subframe as DL, the UE will receive the DL signal in the flexible subframe. Because the flexible subframes can be configured to be in different transmission directions in different cells, they may not fit in the current interference mitigation mechanism.
Interference mitigation (also sometimes referred to as interference cancelation (IC)) is one of the most promising techniques to enhance the performance of wireless access networks, especially for heterogeneous networks and small cells and has been widely discussed in 3GPP. In LTE Rel-11, cell-specific reference signal (CRS)-IC, primary synchronization signal (PSS)-IC, secondary synchronization signal (SSS)-IC, and Physical Broadcast Channel (PBCH)-IC have been standardized for heterogeneous and homogeneous networks. In LTE Rel-11, in order to enable CRS-IC, PSS/SSS-IC, and PBCH-IC, the eNB needs to provide some assistance information, such as number of CRS ports, cell ID, and Multicast Broadcast Single Frequency Network (MBSFN) configuration, to the UE and the UE may utilize this information to cancel CRS, PSS/SSS and PBCH in a network-assisted manner. To enhance UE performance, Physical Downlink Shared Channel (PDSCH) and Physical Downlink Control Channel (PDCCH)/enhanced Physical Downlink Control Channel (ePDCCH) cancelation are under discussion in LTE Rel-12.
Within current scope of network-assisted ICs such as those discussed above, the same UL-DL configurations are assumed to be applied by both the serving cell and aggressor cells, which may be covered by neighboring eNBs that potentially interfere with the serving eNB. Accordingly, the UE can always assume that the DL interference originates from the DL transmissions of the neighboring eNBs. However, such assumption does not always hold true for the dynamic TDD system in which the flexible subframes can be changeably configured to transmit in a UL or DL direction, bringing about additional interference into the dynamic TDD system.
For example, in the dynamic TDD system, the UE in the reception period and served by the serving eNB is likely to experience interference from DL transmission of the neighboring eNB and interference from UL transmission of the another UE served by the neighboring eNB on the flexible subframes. Similarly, in the dynamic TDD system, the serving eNB in the reception period is also possible to be subject to interference from the UL transmission of another UE served by the neighboring eNB and interference from the DL transmission of the neighboring eNB on the flexible subframes. These kinds of inter-UE and inter eNB-eNB interference occurring in the dynamic TDD system cannot be addressed under the current mechanisms for interference mitigation or IC.